Photo-DSC and in situ, time-resolved, laser-induced, steady-state fluorescence spectroscopy were used to study the initiation mechanism of the three-component system: Eosin Y spirit soluble (EYss), N-methyldiethanolamine, and diphenyliodonium chloride. Kinetic studies based on photo-DSC revealed that the fastest polymerization occurred when all three components were present (the next fastest was with the dye/amine pair, and the slowest was with the dye/iodonium pair). However, the laser-induced fluorescence experiments showed that the pairwise reaction between the eosin and iodonium bleaches the dye much more rapidly than does the reaction between the eosin and amine. We concluded that although a direct eosin/amine reaction can produce active radicals in the three-component system, this reaction is largely overshadowed by the eosin/iodonium reaction, which does not produce active radicals as effectively. We proposed that the amine reduces the oxidized dye radical formed in the eosin/iodonium reaction back to its original state as well as the simultaneous production of an active initiating amine-based radical. Because of the difference in the pairwise reaction rates for eosin/amine and eosin/iodonium, it is likely that this regeneration reaction was the primary source of active radicals in the three-component eosin/amine/iodonium system. (C) 2001 John Wiley & Sons, Inc.